The present invention relates to heat exchangers generally, and more particularly to a cooling system suitable for use in conjunction with variously configured air heaters, burner combustion chambers and the like.
Air heaters are often used in conjunction with dryers. For example, a known large industrial slurry dryer is shown in FIG. 1 in conjunction with a ring-shaped type air heater. In such a device, the dryer assembly 1 includes a funnel-shaped housing 2 from which slurry pump and atomizer 3 discharge the slurry from the small diameter end of rotating nozzle 4. The slurry is sprayed into a drying space 5 where the slurry is dried and the dried particles 6 gravitationally drop into a hopper (not shown) for removal. Hot 700.degree.-1000.degree. C. gas-air from plenum 8, flows through annular opening 9, and along the conical outside of housing 2 through conical ring duct 10. The hot gas-air then exits hot air duct 10 as it is directed toward the slurry discharge opening of nozzle 4 where it is mixed with the atomized slurry to drive off the water.
When temperatures in the system illustrated in FIG. 1 exceed about 800.degree. C., which can occur, for example, when a burner is positioned immediately upstream of air duct 10, concerns arise in protecting the duct walls from excessive heat flux. Specifically, mechanical properties (e.g., strength) of the heat resistant steel used to construct the duct walls generally deteriorate at these temperatures. This generally leads to problems in the structural integrity of the system and eventual warping.
Typically, refractory material is used to protect surfaces of air heaters, furnaces, combustion chambers and the like, which are subject to such temperatures, i.e., temperatures in excess of about 800.degree. C. A layer of refractory also protects these surfaces against direct flame exposure. However, the use of refractory has certain drawbacks. Specifically, refractory is susceptible to breakdown and cracking which can result in contamination of the heated air exiting the air heater and, thus, contamination of the product to be dried. Refractory is especially susceptible to cracking when formed in certain configurations such as the conical configuration of the outer wall of housing 2. Thus, although plenum 8 may possibly be lined with refractory, it has been found clearly undesirable to line funnel-shaped walls 11 with refractory material.
When air cooled or water cooled heat exchangers are used to maintain the heater duct walls at a reasonable temperature, concerns arise relating to energy efficiencies. Conventional air cooled systems include a plurality of tubes through which air is forced at a velocity such that there is sufficient heat transfer from the duct wall of the air heater. Generally, these air flow velocities are accompanied with substantially high pressure drops and, thus, relatively high energy consumption.
Energy losses also occur with conventional water cooled heat exchangers (e.g., a water jacket type heat exchanger). In water cooled systems, heat transfers from the working surface, e.g., the duct walls of an air heater, to the water in the heat exchanger. Since the heat lost to the water is not recovered and returned to the air exiting the air heater (such recovery being essentially impractical), energy is lost from the system. In addition, these water cooling systems are undesirable due to high initial and subsequent maintenance costs. For example, these systems generally require expensive water treatment systems to minimize corrosion and plugging of the water conduits.